1. Field of the Invention
The present invention relates generally to magnetic heads for reading data written to storage media, and more particularly to magnetic read heads for disk drives.
2. Description of the Prior Art
A computer disk drive stores and retrieves data by positioning a magnetic read/write head over a rotating magnetic data storage disk. The head, or heads, which are typically arranged in stacks, read from or write data to concentric data tracks defined on surface of the disks which are also typically arranged in stacks. The heads are included in structures called “sliders” onto which the read/write sensors of the magnetic head are fabricated. The slider flies above the surface of the disks on a thin cushion of air, and the surface of the slider which faces the disks is called an Air Bearing Surface (ABS).
The goal in recent years is to increase the amount of data that can be stored on each hard disk. If data tracks can be made narrower, more tracks will fit on a disk surface, and more data can be stored on a given disk. The width of the tracks depends on the width of the read/write head used, and in recent years, track widths have decreased as the size of read/write heads has become progressively smaller. This decrease in track width has allowed for dramatic increases in the recording density of data stored on disks.
A magnetic recording head reads back the information stored as magnetic data bits in the recording medium based on the mechanism that the read sensor's resistance changes with the magnetic field of the data bits. A recording head typically consists of, among other structures, a pinned magnetic layer and a free magnetic layer. The magnetic moment of free layer rotates freely in response to the external magnetic field, e.g., the magnetic field from the recording medium. The magnetic orientation of the pinned layer, by contrast, should be fixed firmly. The magnitude of the read sensor's resistance change is determined by the relative angle between the magnetic moments of the free layer and the pinned layer.
The magnetic moment of the pinned layer is typically fixed by fabricating the pinned layer on an antiferromagnetic (AFM) pinning layer which fixes the magnetic moment of the pinned layer at an angle of 90 degrees to the air bearing surface (ABS).
The free layer material is very soft material, magnetically speaking, with very low coercivity, which is a measure of the minimum field strength necessary to make changes in the orientation of the magnetic domains. The magnetic moment of the free layer is free to rotate laterally within the layer with respect to the ABS from a quiescent or zero bias point position in response to magnetic field signals from data bits located on the rotating magnetic disk. The sensitivity of the sensor is quantified as the magnetoresistive coefficient dr/R where dr is the change in resistance of the sensor from minimum resistance to maximum resistance and R is the resistance of the sensor at minimum resistance.
As referred to above, it is common practice in the art to pin the pinned layer by using a layer of anti-ferromagnetic (AFM) material, which is referred to as the pinning layer. The orientation of the magnetic domains of the AFM pinning layer of the read sensors, and thus the orientation of the pinned layer, is set during the wafer fabrication, typically when the sensor layers are still in the form of continuous thin films. The sensor thin film layers are then processed to their final dimensions.
Magnetization in materials is produced by the alignment of dipoles in the material. Materials in which adjacent dipoles are aligned in the same direction are called ferromagnetic, and are found at room temperature in elements such as iron, nickel, cobalt and gadolinium. The alignment of dipoles results in a net macroscopic moment in the material. In contrast, anti-ferromagnetic (AFM) material has magnetic dipoles which point in opposite directions thus producing no net magnetic moment in the material.
The tendency for neighboring atomic dipoles to line up parallel or antiparallel to each other is called exchange coupling, which results from the overlap of orbiting electrons on adjacent atoms. AFM material when fabricated in contact with ferromagnetic material will tend to lock the orientation of dipoles in the ferromagnetic material by this exchange coupling, as is well known to those skilled in the art. The strength of the exchange coupling is measured by the exchange energy coupling constant Jk, which is measured in erg/cm2. The value of Jk for AFM materials is typically in the range of approximately 0.3-0.4. erg/cm2. The layer of AFM material is typically the thickest layer in the read sensor stack, since a minimum volume of AFM material with Jk in this typical range is required to effectively pin the material in the pinned ferromagnetic layer. Assuming that a minimum value of exchange energy coupling Jk is necessary to securely pin the material in the ferromagnetic layer, then if the Jk value could be increased, i.e. higher erg/cm2, then a smaller volume of AFM material could be used to achieve this required minimum value. The volume, and therefore the thickness of the AFM material layer could be decreased thus allowing for the dimensions of the read sensor and the magnetic head as a whole to be decreased
In fabricating the central sensor stack of a read head, the left and right sides of the sensor stack are defined by ion milling, which thus defines the track width of the sensor. The rear side of the sensor is also defined by ion milling. The front face of the sensor, which faces the recording medium and which will be part of the Air Bearing Surface (ABS), is typically reduced to the operational dimension by a mechanical lapping process. The dimension of the front face to the rear side is known as the stripe height. Both the track width and stripe height are very important to the operating characteristics of the read head and are very tightly controlled during fabrication.
It has been discovered while the pinned layers are well aligned when they are still in the full-film stage on the wafer, they are often mis-aligned when the sensors are reduced to the final dimensions. This misalignment may be caused by damage generated during the ion-milling and lapping process, or be fundamentally due to a re-definition of the boundary conditions of the small volume of magnetic material within the sensor. It is generally true that the smaller the sensor, the more serious is the misalignment. As the recording density becomes increasingly higher, all the dimensions of the read sensor, stripe-height, width and thickness are shrinking, and consequently the misalignment of the pinned layer becomes a more serious problem. An AFM material with a larger value of Jk would pin the ferromagnetic material more securely, and thus would be less vulnerable to damage from ion-milling and lapping.
A paper entitled Giant Exchange Anisotropy Observed in MnIr/CoFe Bilayers Containing Ordered Mn3Ir Phase by Imakita et al, Tohoku University, APL 85,3812 (2004) discloses the production of a Jk value of 1.3 erg/cm2. However, the conditions used therein to produce the depositions are considered not practical for implementation in a fabrication process. Specifically, an ultra high vacuum deposition (10−11 torr) device was used with ultra high purity argon in a DC magnetron sputtering system. The temperature ramp was very rapid, around 250 degrees C. in 10 min with a post annealing temperature of 320 degrees C. The highest Jk values were produced when an L12 ordered phase crystal lattice in the AFM material was formed during repeated annealings. A very thick Cu seedlayer was used, which is considered not practical for magnetic head manufacturing processes.
Thus there is need for a method and structure which increases the exchange energy coupling Jk of AFM material in a manufacturing process, thus pinning the ferromagnetic pinned material more securely and allowing for thinner layers of AFM material to be used, and allowing for the overall volume of the sensor to be reduced.